How to identify Fossiliferous Shale

To correctly identify Fossiliferous Shale (Shale with Trilobita and Brachiopoda fossils), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Typically dark grey, black, dark brown, or greenish-grey, often due to organic content and iron sulfides. The fossils themselves may be lighter in color, contrasting with the matrix, or preserved as dark carbon films.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy on fresh surfaces; may appear slightly waxy or greasy when wet. Fossils may exhibit a slightly different luster depending on their preservation (e.g., calcitic shells might be dull to vitreous).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Very fine-grained, smooth to the touch. Individual grains are generally too small to be seen without magnification. Exhibits fissility, splitting readily along bedding planes into thin, platy fragments. The fossils will be embedded within this fine-grained matrix, often appearing as impressions, molds, or mineralized remains.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for the rock matrix itself, as it is composed of microscopic clastic grains. The fossils, however, retain the external morphology of the original organisms. Brachiopod shells are typically bivalve and symmetrical across their hinge line, while trilobites show a distinct cephalon (head), thorax (segmented body), and pygidium (tail).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Exhibits fissility, a type of parting along bedding planes, rather than true mineral cleavage. This results in the rock splitting into thin, parallel sheets. The fossils themselves do not exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Formed in low-energy marine environments, such as continental shelves, epicontinental seas, or deep-water basins, where fine sediments accumulate slowly and are not disturbed by strong currents. The presence of well-preserved fossils indicates conditions conducive to rapid burial and minimal decomposition, often anoxic bottom waters.

Key Facts

Hardness

2.5-3.5 (Mohs scale for the clay minerals in the matrix; fossils may be harder if calcified or silicified)

Specific Gravity

2.0-2.8 (variable depending on composition and compaction)

Crystal System

Not applicable (clastic sedimentary rock); constituent minerals are typically monoclinic (clays) or hexagonal (quartz)

Color

Dark grey, black, brown, greenish-grey

Luster

Dull, earthy

Transparency

Opaque

Fracture

Splintery, conchoidal (rarely), or irregular across bedding; fissile along bedding

Cleavage

Fissility (parting along bedding planes)

Composition

Predominantly clay minerals (e.g., illite, kaolinite, smectite), quartz silt, and organic matter. Fossils are typically composed of calcite (CaCO3) or chitin (for trilobites, often replaced by calcite or silica).

Physical characteristics

• Crystal Habit: Not applicable for the rock; constituent clay minerals are platy, quartz is anhedral to subhedral.
• Cleavage Type: Fissility (splits into thin, parallel layers along bedding planes)
• Fracture Type: Irregular, splintery, or subconchoidal across bedding; smooth along bedding planes due to fissility.
• Tenacity: Brittle
• Luster Type: Dull to earthy

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